2013 Seventh International Conference on Sensing Technology
Individual Nanoparticle Zeta Potential Measurements
using Tunable Resistive Pulse Sensing
Eva Weatherall, Geoff R. Willmott
Ben Glossop
Callaghan Innovation
Lower Hutt, New Zealand,
and The MacDiarmid Institute
SCPS, Victoria University of Wellington, New Zealand
Email: [email protected]
Izon Science
8C Homersham Place, Burnside
Christchurch, New Zealand
particle-by-particle resistive pulse measurements were first
reported by DeBlois et al. [9], who used polycarbonate
nanopores to obtain zeta potentials for populations of 91 nm
latex spheres and a range of viruses, calculating the particle
mobility from the measured resistive pulse rate. Ito et al. [1012] later reported zeta potentials for 60 nm carboxylated
polystyrene particles based upon the total duration of
individual resistive pulses, measured using carbon nanotubebased Coulter counters. Both of these groups worked with
cylindrical pores of fixed size.
Abstract- Tunable resistive pulse sensing has been used to
measure the zeta potential of two sets of 200 nm diameter
polystyrene nanoparticles in 0.1 M KCl electrolyte. Data were
analysed using two methods, both of which yield particle-byparticle zeta potential values based on electrophoretic mobility.
Five pore specimens were used, and in most cases data clearly
indicate that one particle set had a higher (less negative) zeta
potential than the calibrated zeta potential of the other particle
set (-35 mV). Three tungsten needles were used to fabricate the
pores, and comparable results were obtained for pores made by
the same needle. Measurements were carried out with varying
potential applied, and a reliable working range of 0.3 to 0.6 V
was observed. Measurement of zeta potentials on individual
nanoparticles is important for colloidal applications such as
foods, cosmetics, mineral extraction and biomedical technologies.
These studies have been improved upon in recent analyses
of resistive pulse-based zeta potential measurement [13, 14]. In
particular, a semi-analytic approach has been developed for
TRPS [4, 6-8, 15-19], which uses knowledge of the dimensions
of conical pores along with key transport mechanisms
(pressure, electrophoresis and electro-osmosis) to model
resistive pulse frequency, size and shape as a function of
particle characteristics. Vogel et al. [7] first used TRPS to
demonstrate zeta potential measurement using resistive pulse
rates. Pressure driven flow was precisely controlled in order to
equilibrate transport mechanisms acting on the particles,
enabling identification of a point of minimum pulse frequency,
and consequent calculation of zeta potential. This method was
subsequently used to measure the zeta potential of oil-in-water
emulsion droplets [20].
Keywords- zeta potential; nanoparticle; electrophoretic
mobility; tunable resistive pulse sensing; Coulter counter
I.
INTRODUCTION
Tunable resistive pulse sensing (TRPS) is a single
nanoparticle analysis technique which combines resistive pulse
sensing, a technique based on the Coulter principle, with sizetunable pores (Fig. 1). The diameter of these size-tunable
pores, which are manufactured by mechanically puncturing a
pore into an elastic membrane [1], can be altered by radially
stretching or relaxing the membrane. The tunability gives each
pore an increased analysis size range, improved measurement
sensitivity and introduces the possibility of gating larger
particles [2]. TRPS has been shown to provide excellent
resolution and accuracy for measuring size distributions of
polydisperse particles, surpassing measurements made using
ensemble techniques [3]. As well as measuring size [4],
previous studies have used size-tunable pores to measure
particle concentration [5, 6], and to infer information about
particle surface charge [6-8].
Particle surface charge, an important property of colloidal
dispersions that gives information about particle composition
and dispersion stability, is usually characterized in terms of a
zeta potential, inferred from particle electrophoretic mobility.
Conventional zeta potential measurement techniques (e.g.
phase analysis light scattering) rely on ensemble measurements
which are inherently sensitive to subpopulations of larger
particles in a sample. Zeta potential measurements using
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Figure 1. Apparatus for TRPS experiments. Left: the qNano, which is ~30
cm tall and holds a pore specimen (right) in a fluid cell at the top of the
device. The stretch X applied to the pore specimen (resting value 42 mm)
is measured between sets of teeth placed in holes at the ends of opposite
arms on the specimen, and is symmetric across the two pairs of arms. An
instrument for applying variable pressure to the fluid cell (the black
cylinder pictured with the qNano) was not used in the present work.
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2013 Seventh International Conference on Sensing Technology
Figure 2. Scanning electron microscopy of tungsten needles used to make pores. Left to right, needles A, B and C. The scale bar (1 µm) applies to all images.
For all TRPS experiments, 40 µL of the particle suspension
was added to the upper half of the qNano’s fluid cell and 70 µL
was added to the lower half. An ionic current was driven and
recorded by application of a potential V0 across the two halves
of the cell (Fig. 3 inset). The analog digital converter operates
at 1 MHz, which is reduced to a sampling rate of 50 kHz
through electronic filtering. The current pulse signals were
collected and analysed using IZON proprietary software (v2.4).
A minimum of 200 events were recorded for each
measurement condition. Experiments for Method 1 (see
Section III) were calibrated at 3 stretches, with constant applied
potential, to achieve currents between 80 and 130 nA. Method
2 experiments were carried out at a single stretch chosen to
give a current of 120-130 nA at an applied voltage of 0.6 V.
Kozak et al. [17] built position-in-time (velocity) profiles
for individual particles based on experimental data. With the
total pressure applied across the pore still precisely controlled,
the zeta potential of individual particles could be calculated.
This analysis method was able to successfully resolve a
complex bimodal suspension, composed of two differently
charged 300 nm polystyrene particle sets [8]. Here, we further
investigate the use of single particle zeta potential analysis
methods in practice. A method similar to Kozak et al.’s
technique, in which the entire resistive pulse data trace is
analysed, is compared with a simplified method examining
discrete points along the resistive pulse. The effects of voltage
and specimen fabrication on zeta potential measurements are
explored using both methods.
II.
MATERIALS AND METHODS
TABLE I.
A. Particles and Solutions
Carboxylated polystyrene particle standards with nominal
diameter 200 nm and surface group titration of 86.0 µeq g-1,
purchased from Bangs Laboratories (USA) and supplied by
Izon Science (Christchurch, NZ), are referred to here as
charged particles. These particles were used for charge
calibration, with a zeta potential of -35 mV defined by the
supplier. Uncarboxylated polystyrene particles (NIST traceable
size standards) with nominal diameter 200 nm, referred to here
as uncharged particles, were purchased from Thermo Fisher
(USA). In all TRPS experiments, the electrolyte used consisted
of 0.1 M KCl, 15 mM tris(hydroxymethyl)aminomethane
(Tris) buffer, 0.01% v/v Triton X-100, and 3 mM
ethylenediaminetetraacetic acid (EDTA) in deionised water.
Nanoparticles were immersed in electrolyte at concentrations
of approximately 109-1010 particles mL-1.
Pore
Needle
A (µm)
B (µm)
L (µm)
(i)
A
0.75
55.13
266.40
(ii)
A
0.70
47.43
266.40
(iii)
B
0.56
63.05
266.40
(iv)
C
0.68
88.71
257.58
(v)
C
0.82
114.63
248.58
III.
DATA ANALYSIS METHODS
A. Method 1
The first method for calculating zeta potential from individual
resistive pulses has been developed and demonstrated
previously [8, 17]. This method uses a defined pore geometry
to calculate particle position-in-time for each resistive pulse.
Pores are assumed to be truncated linear cones, with openings
of diameter A and B at the surfaces of a membrane of
thickness L (Fig. 3 inset). Each of these dimensions changes
with stretch, and L has been pre-characterized as a function of
X for the type of membrane used in our work [17].
B. Tunable Resistive Pulse Sensing (TRPS)
Tunable pore specimens obtained from Izon Science are
fabricated in thermoplastic polyurethane membranes as
described previously [1]. In this study, we used five pore
specimens (Table 1) which were fabricated using three
different needles (Fig. 2). Pore specimens were mounted on the
stretching jaws of the qNano device (Fig. 1, Izon Science)
which enabled stretching. The applied stretch is quantified by
the distance (X) between opposing jaws, 42 mm at rest.
Specimens were pre-stretched to 49 mm for 5 minutes prior to
experimental use.
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PORE SPECIMENS, AND DIMENSIONS FOR METHOD 1.
A is calculated by measuring ΔR, the maximum resistance
change from the baseline value R0 (Fig. 3), for resistive pulses
generated by a particle set of known diameter d. Assuming that
the pore diameter near the particle is A when this maximum
occurs, then
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2013 Seventh International Conference on Sensing Technology
defining the pore geometry. In Method 2 it is assumed that for
any two particles under the same experimental conditions, the
particle position will be the same when the fraction of ΔR is
the same. The average vp (Eq. 2) is determined using data
obtained over the 0.2-0.6 V range of applied potentials for the
calibration particle set of known ζpart. For a resistive pulse
generated by a particle of unknown zeta potential, the
measured v, along with the known vp, are used to calculate vel
(Eq. 2). The particle zeta potential is then calculated using
Eq. 3, by comparison of vel with the calibration particles of
known ζpart, and the known value ζpore.
IV.
RESULTS AND DISCUSSION
Fig. 4 plots particle zeta potential measurements as a
function of applied potential, with data obtained from
experiments using the two particle sets and five pore specimens
(see Section II). Calculations were performed using both
analysis methods from Section III.
Figure 3. Schematic plot of a resistive pulse, with the maximum change in
resistance (ΔR) from the baseline level (R0 = V0 / I0) occurring at time t0.
Inset, schematic section of a particle passing along the axis of a conical
pore, with geometric variables indicated.
A = (4ρd3 / πΔR)0.25,
The charged particle set was used for calibration of both
methods, with an assigned zeta potential of -35 mV.
Calibrations were carried out at 0.6 V applied potential for
Method 1 and 0.2-0.6 V for Method 2. Because the charged
particle sets were used for calibration, it is expected that the
zeta potential values for these particles should be -35 mV,
indicated by the horizontal dashed lines in Fig. 4. For all pores
and both analysis methods, results were consistently within
experimental error of this value when the applied potential was
in the range 0.3 to 0.6 V.
(1)
where ρ is the electrolyte resistivity.
To establish a relationship between the applied stretch and
pore opening A as in [17], resistive pulse measurements were
obtained for particles of known diameter at 3 applied stretches,
1 mm apart. The larger pore opening B, and its variation with
stretch, were then calculated using calibration particles with an
assigned zeta potential. In this way, for each pore specimen
used the geometry (A, B and L) is defined at a given applied
stretch. The dimensions determined for the pores used in this
study, at the reference stretch for calibration, are reported in
Table 1.
When using both methods, charged and uncharged particles
could be distinguished using pores (i) and (ii), manufactured
using Needle A, and pore (iii), manufactured using Needle B.
However, measurements from pores (iv) and (v), manufactured
using Needle C, revealed little difference in zeta potential
between particle sets within experimental error.
Overall, the two analysis methods produce similar results,
but it appears that Method 2 produces a larger spread of data.
For example, the zeta potential on the uncharged particle set
appears higher (more positive) for all pores when using
Method 2. For pore (v), the median values for charged and
uncharged particles are consistently distinguishable using
Method 2, unlike Method 1, and the spread of data was also
greater using Method 2. It is proposed that this difference is
due to the sampling of discrete points along the resistive pulse
in Method 2, as opposed to fitting to the entire pulse in Method
1, giving rise to greater variability in the data.
With the pore geometry established, particle position-intime is calculated using data after time t0 for each experimental
resistive pulse (Fig. 3), corresponding to when the particle is
entirely within the pore, in order to avoid end effects. The
velocity of particles within the pore can be expressed as [7]
v = vel + vp,
(2)
where vel and vp are, respectively, electrokinetic and pressuredriven contributions to transport, and
vel = εE(ζpart – ζpore) / η.
(3)
A key reason for variability in the results obtained using
different pores is geometric variation from specimen to
specimen, intentional or otherwise. The pore geometry is
dependent on the characteristics of the tungsten needle,
puncture control and the material properties [21]. Pores
manufactured using the same needle, similar penetration
settings and the same membrane material are therefore
expected to exhibit similar properties, as observed in Fig. 3.
Needles A and B are the sharpest of the 3 needles whereas
Needle C can be seen to be much broader (Fig. 2). It is
therefore proposed that the geometry of pores manufactured
using sharper needles is closer to the model used for
Here ε and η are the electrolyte dielectric and viscosity
respectively, E is the magnitude of the electric field, and ζpart
and ζpore are the zeta potentials of the particle and the pore wall
respectively. With the geometry known and ζpore determined
for the pore material [7, 8, 20], ζpart is the sole unknown, and
can be varied to fit the experimental data using a least squares
method [8].
B. Method 2
Method 2, which also examines only times following t0, is
simplified in order to remove the complication of specifically
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2013 Seventh International Conference on Sensing Technology
Figure 4. Zeta potential values for charged (“CH”) and uncharged (“UNCH”) particle sets, derived using two analysis methods (Section III), plotted as a
function of potential applied across the membrane. The five pores used were fabricated by three different needles (Table I). Dashed horizontal lines indicate
the calibration value for charged particles at 0.6 V applied potential (-35 mV). Error bars indicate the upper and lower quartile values for each data point.
calculations, being perhaps more regularly shaped and closer to
a truncated linear cone (Fig. 3), leading to increased precision
and accuracy in the results.
difficult to define. Finally, we note that for true particle-byparticle zeta potential measurement (as opposed to an ensemble
average), the contribution of electrophoretic mobility to
resistive pulse shape must be distinguished from other factors,
such as off-axis trajectories, Brownian motion, steric
interactions, and electronic sampling effects [20].
Results are more variable and less consistent at applied
potentials below 0.3 V and above 0.6 V, defining a working
range for apparatus, pores, applied stretches and particles
similar to those we have used. At low potentials, the
electrokinetic component of particle motion becomes
dominated by pressure-driven flow, so that the electrophoretic
mobility of each particle is difficult to discern. At high
potentials, electronic noise increases.
V.
Two methods have been demonstrated for analyzing
resistive pulses to obtain zeta potential data on a particle-byparticle basis. Results are most relevant to tunable resistive
pulse sensing of 200 nm particles in 0.1 M KCl, with
0.3 - 0.6 V applied across the pore. Overall, the two methods
produced similar results, clearly distinguishing between two
sets of nanoparticles with different zeta potentials in most
cases. The second method appears to be superior for
distinguishing particles when the pore geometry is difficult to
define. Although measurement variations were observed for
different pore specimens, pores made using the same tungsten
needle produced comparable results. Ongoing work will
address underlying assumptions and apply the techniques
described to a broader range of nanoparticulate systems.
As an overall perspective on the two analysis methods, the
major challenge for Method 1 is definition of the pore
geometry, and the change in geometry as the pore is stretched
or relaxed. As defined above, Method 1 differs slightly from
the original methodology proposed by Kozak et al. [8, 17],
because a calibration particle set of known zeta potential is
used to define B, rather than using the baseline current. This
change reduces the calculation’s dependence on specific pore
geometry, but adds the requirement for calibration particles.
Method 2, which also requires calibration particles, removes
the need for well-defined pore geometry, instead making an
assumption regarding the similarity of resistive pulse shapes
for similar experimental conditions. Although the accuracy of
this assumption can be explored further, our data for pore (v)
suggest that Method 2 is superior for distinguishing particles of
different zeta potential when the geometry of the pore is more
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CONCLUSION
ACKNOWLEDGMENT
We thank Izon Science for provision of specimens, SEM
imaging and supporting experiments.
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2013 Seventh International Conference on Sensing Technology
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